Exposure of Cs surfaces to O2 causes ejection of O- ions with low yields (∼10-8 per incident O2 molecule) during the first stages of dissociative chemisorption (followed by exoelectron emission at higher exposures), although the work function of the surface exceeds the electron affinity of O and the energetics of the overall reaction is almost zero. A mechanism is proposed whereafter the release of O- is a consequence of strong repulsion in O22- species intermediately formed in front of the surface.
